This invention relates to switching circuits, and in particular to circuits including a pair of switching elements each of which can be caused by an input pulse to conduct for an output pulse.
Problems can arise if, owing to differences between the switching elements, the output pulses for the two elements are unequal in duration. An example occurs when the switching elements are semiconductors connected in push-pull configuration and driving an output transformer, and the circuit is operated as the inverter stage of a power supply. In that case differences in the durations of the output pulses can drive the transformer into saturation, with increased and possibly destructive dissipation in one of the semiconductors.
An important factor influencing differences in conduction time in semiconductors is the storage time, which results from the presence of injected minority carriers in the base or gate region of the semiconductor at the time when the input current is cut off. One way of overcoming the problem is by carefully selecting the semiconductors as matched pairs, with equal storage times. However, such selection is time-consuming and often not completely satisfactory since the storage times of the semiconductors tend to drift by different amounts with age and temperature. It has also been proposed to control the duration of the input pulses by a feed-back loop which compares the output pulses from the two semiconductors and alters the ratio between the input pulses to the two semiconductors in such a way as to tend to make the output pulses the same.